Temperature Control in Distillation Tower

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The temperature decreases as the material move higher in the tower. The gradual decrease in temperature from the bottom of the distillation tower to the top is called the temperature gradient. The temperature gradient is measured in term of difference between the temperature at the bottom of the tower and the temperature at the top of the tower.

In order to maintain the proper temperature gradient, temperature at the critical point in the system must be controlled. Temperatures in the distillation system are typically controlled in three ways.

Pre-heater   
One way is to control the temperature of the mixture by using a pre-heater, this regulate the temperature at the feed point. At the bottom of the tower temperature is controlled by the amount of heat that is added by re-boiler, this added heat is referred to as boil-up. The temperature at the top of the tower is controlled by the amount or the temperature of the cool liquid that is pumped back in the tower or from the over head receiver and this is called the reflux rate. Increasing the reflux rate decreases the temperature of the top of the tower.

Pump around
Some distillation system contains equipment known as pump around. The purpose of pump around is to remove hot liquid from the tower and pumps it to a cooler. The cool liquid is then re-introduced at higher level in the tower. A pump around help controls temperature of the internal reflux.

Pressure
Since pressure effect the boiling temperature of the liquid, it is an important factor in the distillation process. If pressures in the distillation are not correct, product purity could decrease. Tower pressure is often controlled by pressure control valve located on the overhead receiver. This valve controls pressure by releasing vapor and any non condensable gasses that are collected in the receiver. In some cases, a vacuum system draws gases from the receiver to control tower pressure.

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